Data communication channels are used to transmit and receive information in an efficient and reliable manner, Such channels are utilized in a variety of applications from wireless communication networks, such as mobile cellular and satellite communication systems, to computer data storage devices, such as hard disc drives.
In a receiving portion of a data communication channel, detection circuitry is necessary to detect and decode the information being transmitted, One basic type of detection circuitry is known as a detection feedback equalizer (DFE). As will be recognized by those skilled in the art, a typical DFE comprises a slicer which applies a selected threshold to a prefiltered input signal to generate a corresponding decision nominally indicative of the data value of the input signal (such as +1 or -1). A feedback filter having a response excited by past decisions provides an output which is subtracted from the input signal in order to cancel trailing intersymbol interference (ISI), with the resulting sum characterized as a decision variable. An error term is also generated in relation to the presence of noise in the channel as a difference between the decision variable and the decision.
In the absence of noise, the decision variable will generally be equal in magnitude to the input signal, the decision will be correct and the error term will be zero. However, as the amount of noise in the channel increases, the error term will grow; when the noise is sufficiently large, the decision variable will change polarity and an erroneous decision will be made. This erroneous decisions can propagate for some time due to the feedback provided by the feedback filter. For examples of communication channels utilizing various DFE based architectures, see U.S. Pat. No. 4,985,902 issued Jan. 15, 1991 to Gurcan, U.S. Pat. No. 5,027,369 issued Jun. 25, 1991 to Kuenast, and U.S. Pat. No. 5,430,661 issued Jul. 4, 1995 to Fisher et al.
To provide improved error rate performance over a single DFE, a dual decision feedback equalizer (DDFE) architecture was recently discussed by Bergmans et al. in a paper entitled "Dual Decision Feedback Equalizer," Philips Research Laboratories, Prof. Holstlaan 4, 5656 Eindhoven. The Netherlands, Nov. 27, 1996. In this paper, Bergmans et al, propose the use of two DFEs which are operated in parallel so that each independently makes decisions on an input sequence. The DFEs are nominally identical, except that the first DFE utilizes a slicer threshold of .alpha. and the second DFE utilizes a complementary slicer threshold of -.alpha. (the interval -.alpha. to .alpha. defining an "erasure zone").
When noise is small, the decisions of both DFEs will be correct and identical. However, when one or both of the decision variables fall within the erasure zone, the first DFE is caused to output a decision +1, the second DFE is caused to output a decision -1, and an erasure flag is set. At this point the slicer thresholds are both temporarily set to zero. The respective error terms are then accumulated for a selected amount of time during an erasure period and the DFE with less error energy is selected as the correct sequence of decisions for the period. This is based on the reasoning that the erroneous decision sequence will likely contain a larger error energy than the error energy associated with the correct decision sequence. At the conclusion of the erasure period, the slicer thresholds are reset and the DFEs resume independent operation.
A DDFE provides improved error rate performance over a DFE, approaching the higher rates of performance achievable by more complex types of detectors, such as the well-known Viterbi type. Improvements in the error rate performance of a DDFE are nonetheless desirable, especially for high data rate transfer applications such as disc drives, and it is to this end that the present invention is directed.